1. Field of the Invention
The present invention relates to a high-capacity cellular network and, more particularly, to a method and apparatus for a high-capacity cellular network by improved sectorization and interleaved channel assignment.
2. Description of the Art
Capacity is one of the most important issues in wireless systems. Because of the limited available frequency spectrum, current cellular radio systems adopt the concept of frequency reuse to utilize the same frequency repeatedly at different locations. A large frequency reuse distance can enhance channel quality due to low interference, but will decrease the overall system capacity. One challenge for cellular engineering, then, is to optimize the tradeoff among channel quality, system capacity, and the costs of infrastructure and user terminals.
In an attempt to meet the challenge, there are two directions taken to improve the tradeoff between channel quality and system capacity. One direction is to adopt more sophisticated technologies, such as code division multiple access (CDMA), adaptive antenna array, and dynamic channel allocations (DCA), etc. These techniques are capable of handling high interference, thereby reducing frequency reuse distance and thus increasing system capacity. They also relieve the burden of frequency planning. However, in addition to increasing the cost of base station equipment and user terminals, these techniques also breed a host of new issues. For example, a CDMA system requires sophisticated power planning to achieve high capacity. The adaptive antenna array processing needs to deal with the power consumption issue and the size of users"" handsets. The DCA systems must meet some operational conditions to function effectively, e.g., the accuracy of time synchronization among all base stations and the agility of user terminals"" synthesizers, etc.
On the other hand, the second direction, a more economical approach to enhance the spectrum efficiency, is to develop a better cellular engineering methodology. This approach is economical in the sense that it minimizes the cost of base station equipment and requires no changes on user terminals at all. Thus a better cellular engineering methodology usually results in equivalent improvements on both downlink and uplink transmissions. This type of cellular engineering includes three major aspects: 1) optimizing frequency planning to reduce interference; 2) selecting a cell architecture to improve the coverage and interference performance; and 3) choosing better cell site locations to enhance service coverage.
To optimize frequency planning and coverage and interference performances, traditional cellular engineering considers a frequency reuse factor of Nxe2x89xa73, only to ensure at least a xe2x80x9cbufferedxe2x80x9d cell between co-channel cells, where the re-use factor N is defined as the number of cells repetitively sharing the whole frequency spectrum once. The smaller the reuse factor N, the closer together the same channel frequency can be utilized at different antenna locations in the cellular network and, thus, the higher the system capacity. Few papers in the area of cellular engineering even discuss system architectures with a low reuse factor of Nxe2x89xa62. One paper, an article entitled xe2x80x9cA Novel Two Site Frequency Reuse Planxe2x80x9d by J. Xiang, discusses a cell planning approach that can achieve a reuse factor of N=2, unfortunately, however, at the cost of using six antennas at a cell. Also, in an article entitled xe2x80x9cRadio Resource Allocation in Fixed Broadband Wireless Networksxe2x80x9d by T. K. Fong et al., a frequency planning method with a reuse factor of N=1 is discussed, but is, however, only suitable for receiver terminals at fixed locations. Lastly, an article entitled xe2x80x9cA New Cellular Architecture based on an Interleaved Cluster Conceptxe2x80x9d by Li-Chun Wang, and which is hereby incorporated by reference, discusses that the cellular system can achieve a reuse factor of N=2 with good channel quality by using a sector rotation technique and clover-leaf cellular architecture. However, with this technique the impact of variations of cell site location are unknown.
Accordingly, the present invention introduces an improved cellular planning method and apparatus to achieve a high-capacity cellular mobile network having a reuse factor as low as N=2. Such a low reuse factor, while maintaining coverage and interference performance optimization, is achieved by an improved cellular architecture, the Narrow-Beam Quad-sector Cell (NBQC), in conjunction with a new frequency planning technique, the Interleaved Channel Assignment (ICA).
The Narrow-Beam Quad-sector Cell (NBQC) is a sectorization scheme for a cell of a cellular network which employs four 60xc2x0 directional antennas at a base station, each of which is separated by 90xc2x0. Such a sectorization scheme thus breaks up the single cell into four square-shaped sectors and allows for better coverage and interference performance.
Taking advantage of the NBQC, the Interleaved Channel Assignment (ICA) is a unique way of channel assignment in which each cell in the same column of a cellular network is assigned with four channels (or channel sets), one for each of the cell""s four sectors. To take full advantage of the directivity of the sectoral antennas, the channels assigned to the corresponding sectors of adjacent cells in the same column are interleaved. Thus the interleaved channel assignment allows for cells in a neighboring column to use a different set of four channels, thus yielding a frequency reuse factor of 2 and optimizing frequency planning.
The present invention, including its features and advantages, will become more apparent from the following detailed description with reference to the accompanying drawings.